235
Indonesian Journal of Science & Technology 6 .
235-242 Indonesian Journal of Science & Technology Journal homepage: http://ejournal.
edu/index.
php/ijost/ Carbon-coated Single-phase Ti4O7 Nanoparticles as Electrocatalyst Support Christina Wahyu Kartikowati1.
Aditya Farhan Arif2*.
Osi Arutanti3, and Takashi Ogi4 Department of Chemical Engineering.
Universitas Brawijaya.
Indonesia Department of New Investment.
PT.
Rekayasa Industri.
Indonesia Research Center of Chemistry.
Indonesian Institut of Science.
Indonesia Department of Chemical Engineering.
Hiroshima University.
Japan Correspondence: aditya_farhan@rekayasa.
ABSTRACT
The unique structure of Magnyli phases TiO x renders them effective for the electrochemical applications.
This work demonstrates a synthesis of carbon-coated Magnyli phases TiOx (TiOx@C) nanoparticles from 3-aminophenol and rutile titania (TiO.
nanoparticles as a support for platinum (P.
3-aminophenol was polymerized and carbonized on the surface of TiO2 nanoparticles respectively in a microwave hydrothermal reactor and a tubular furnace.
Reduction of the carbon-coated TiO2 (TiO2@C) into TiOx@C was performed in hydrogen atmosphere at 800-1050 AC.
The carbon coating effectively prevented TiO2 nanoparticles from sintering, resulted in TiOx@C sizes from 50 to 100 nm.
Singlephase Ti4O7 core, which has the highest theoretical electrical conductivity among the Magnyli phases, was obtained from reduction of TiO2@C at 1000 AC.
for 10 min C/Ti4O7supported Pt exhibited an electrochemical surface area of 46 m2 mgPt-1 at 15% Pt loading, slightly higher than that reported for commercial TKK electrocatalyst with 20% Pt loading .
13 m2 mgPt-.
A 2021 Tim Pengembang Jurnal UPI
ARTICLE INFO
Article History:
Submitted/Received 21 Nov 2020
First revised 03 Jan 2021
Accepted 05 Mar 2021
First available online 08 Mar 2021
Publication date 01 Apr 2021
____________________
Keyword:
Magnyli phase.
Substoichiometric.
Core-shell.
Nanoparticle.
Hydrothermal Kartikowati, et.
Carbon-coated Single-phase Ti4O7 Nanoparticles as Electrocatalyst Support | 236
INTRODUCTION
Magneli phases titanium oxide with a chemical formula of TinO2n-1, or simply called Magneli phases TiOx, are attractive because of their unique structure.
The oxygen deficiencies form an ordered crystallographic shear of Ti2O3 corundum, which is continuously introduced throughout the rutile TiO2 phase (Nyrenberg, et al.
, 1.
The interfacing Ti atoms between these two phases share electrons in the 3d orbital.
The crystallographic shear enables a high theoretical electron conductivity, of which value depends on the crystal arrangement.
was reported that the highest theoretical electron conductivity may reach 1000 S cm-1 (Li, et al.
, 2.
This value is achieved by Ti4O7 which consists of seven Ti-O bonds for every Ti-Ti bond with a very well-ordered crystallographic shear plane as the electron pathway (Phillips, et al.
, 2.
Development of Ti4O7 materials has been significantly improving in the recent years.
This is consistent with the increasing demand for conductive materials.
Besides having high electron conductivities.
Magneli phases are generally stable under strongly oxidizing environment (Esfahani, et al.
, 2.
This electrochemical system involving acidic environment and potential cycling with a wide potential window.
Ioroi, et al.
demonstrated this material as a stable, durable, and highly efficient electrocatalyst support for platinum (P.
(Ioroi, et al.
, 2005
and Ioroi, et al.
, 2.
Although the performance of Ti4O7 as electrocatalyst support has been proven, morphological structure of this material is still an issue.
It is generally accepted that homogenous structure with small size is highly favored for this application.
Ti4O7 is mainly synthesized using a topdown approach, i.
by reducing TiO 2 at high temperatures (> 1000 AC) in hydrogen or The reduction temperatures exceed the sintering temperature of TiO2, resulting in a big size of products which is unfavorable for most of the applications, including electrocatalyst support.
Previous studies showed that Ti4O7 with small size could be synthesized using bottom-up approaches, i.
by controlling the growth of Magneli phases.
One good example of this approach is a study by Phillips, et al.
which used atomic layer deposition to grow a single phase Ti4O7 (Phillips, et al.
, 2.
However, this method required a substrate for the Ti4O7 phase to Recently.
Arif et al.
introduced the first synthesis of nano-sized Magneli phases TiOx with chain-structure using induction thermal plasma method (Arif, et al.
, 2.
Although this method did not require any substrate and successfully decreased the particle size down to 20 nm, control of the crystal structure to form a single Ti4O7 phase was a challenging issue.
The presence of Magneli phases other than Ti4O7, as well as some conductivity far below the expected value.
This is to say, that a synthesis of single phase Ti4O7 with nano size remains a challenge.
A possible approach to solve this challenge is using a top-down approach which features anti-sintering agent to encapsulate TiO2 before reduction at high temperatures.
Carbon is a good anti-sintering candidate for the production of Ti4O7 because of two First, carbon is conductive and therefore, is able to maintain high Second, the structure of carbon can be controlled to be sufficiently porous.
The porous structure enables the penetration of the reducing gas through carbon layer to reach and reduce TiO2 core.
Based on this rationale, the present study demonstrates for the first time, a synthesis of single phase, nano-sized carbon-coated Ti4O7 (Ti4O7@C) from rutile TiO2 and 3aminophenol.
TiO2 was encapsulated with 3aminophenol-derived carbon prior to the reduction at high temperatures.
The encapsulation was assisted by microwave irradiation using a previously demonstrated method (Arif, et al.
, 2.
After the DOI: https://doi.
org/10.
17509/ijost.
p- ISSN 2528-1410 e- ISSN 2527-8045 237 | Indonesian Journal of Science & Technology.
Volume 6 Issue 1.
April 2021 Hal 235-242 successful reduction into single phase Ti 4O7.
Pt nanoparticles as electrocatalyst were deposited on Ti4O7@C using a microwaveassisted The electrochemical surface area (ECSA) of the Pt/Ti4O7@C electrocatalyst was then evaluated as the performance parameter.
The results demonstrated in this study are expected to be the basis for further Ti4O7@C electrocatalyst support in the future.
previous report (Balgis, et al.
, 2.
10 mg of Ti4O7@C nanoparticles were dispersed in 20 ml ethylene glycol (Kanto Chemical Co.
Inc.
Tokyo.
Japa.
Chloroplatinic acid was added and the solution was further ultrasonicated for 30 The solution was subjected to microwave irradiation for 2 min at 150 AC.
The obtained particles.
Pt/Ti4O7@C, were washed several times with water and ethanol, and dried.
METHODS
Synthesis of Ti4O7@C nanoparticles 3 Characterization 1 g of TiO2 nanoparticles (Sakai Chemical Industry.
Co.
Ltd.
Osaka.
Japa.
, rutile phase, were dispersed in a mixture of ethanol .
69 m.
, ammonia .
44 ml.
Kanto Chemical Co.
Inc.
Tokyo.
Japa.
, and ultrapure water .
5 m.
in an ultrasonic bath.
1 g of 3aminophenol (Sigma Aldrich.
St.
Louis.
MO.
USA) were added into the dispersion and stirred until dissolved.
The mixture was then subjected to microwave irradiation in a microwave reactor (Initiator .
Biotage.
Uppsala.
Swede.
for 30 min at 150 AC after the addition of formaldehyde (Kanto Chemical Co.
Inc.
Tokyo.
Japa.
The obtained particles were separated from the remaining solution using centrifugation and washed several times with water and After being dried at 40 AC, the particles were carbonized in a ceramic furnace for 2 hours at 1000 AC in nitrogen The resulting particles.
TiO2@C, was then placed in a quartz tube reactor for reduction in continuously flowing .
L min -.
hydrogen (H.
for 10 min.
Before the reduction started, the oxygen content in the reactor was kept minimum by evacuating the air and filling the reactor with high purity H 2.
The effect of reduction temperature was studied at 800, 900, 1000, and 1050 AC.
2 Pt deposition on Ti4O7@C nanoparticles Deposition of Pt on Ti4O7@C nanoparticles followed the procedure described in the Crystal structure of the prepared particles was analyzed using x-ray diffraction (XRD.
Bruker D2 Phaser.
Bruker AXS GmbH.
Karlsruhe.
German.
The phase composition was evaluated semi-quantitatively in Diffrac EVA 3.
0 software (Bruker AXS GmbH.
Karlsruhe.
German.
The morphology was observed using a fieldemission scanning electron microscope (SEM.
S-5000, 20 kV.
Hitachi High-Tech Corp.
Tokyo.
Japa.
and transmission electron microscope (TEM.
JEM-2010, 200 kV.
JEOL Ltd.
Tokyo.
Japa.
The -potential of TiO2 and the polymer was measured using a zetasizer (Zetasizer Nano ZSP.
Malvern Instruments Ltd.
Malvern.
UK).
Electrochemical characteristic of the prepared particles was evaluated using a potentiostat (Hz-5000.
Hokuto Denko Corp.
Tokyo.
Japa.
in a 3-electrode configuration.
The counter and reference electrode were respectively platinum wire and reversible hydrogen electrode (RHE).
The catalyst ink was prepared by dispersing 2.
64 mg of Pt/Ti4O7@C in 2-propanol .
3 ml.
Kanto Chemical Co.
Inc.
Tokyo.
Japa.
, ultrapure water .
95 m.
and Nafion .
AAl.
Wako Pure Chemical Industries Ltd.
Osaka.
Japa.
in an ultrasonic bath.
The catalyst ink .
AA.
was transferred to a polished glassy carbon electrode and allowed to dry.
Cyclic voltammetry was performed without rotation in a potential window between 0 2 V/RHE with a 50 mV s-1 potential DOI: https://doi.
org/10.
17509/ijost.
p- ISSN 2528-1410 e- ISSN 2527-8045 Kartikowati, et.
Carbon-coated Single-phase Ti4O7 Nanoparticles as Electrocatalyst Support | 238 The electrolyte 1 M HClO4.
RESULTS AND DISCUSSION
The first part of the synthesis of Ti4O7@C nanoparticles was encapsulation of TiO2 with The encapsulation mechanism was expected to be similar with that described in the previous report, except that TiO2 was used as the foreign body the current study (Arif, et al.
, 2.
Initially, 3-aminophenol monomer nucleated on the surface of TiO2.
The driving force for the nucleation was the electrostatic attraction due to the opposing -potential.
-potential of TiO2 was observed to be -30 mV.
This value was contributed mainly by the OH- group on the particle On the other hand, 3-aminophenol was positively charged .
mV) which reflected the presence of amine group in the Polymerization of 3-aminophenol was initiated immediately after the addition of formaldehyde (Zhao, et al.
, 2.
The polymerization took place in the microwave At the end of the polymerization.
TiO2 nanoparticles were covered with 3aminophenol polymer layer, creating a coreshell structure with the core being TiO 2 The polymer was then converted to carbon after carbonization and the whole particle became TiO2@C.
The mechanism of TiO2 encapsulation with carbon is illustrated in Figure 1.
The prepared TiO2@C nanoparticles were then reduced in H2 at high temperatures to form TiOx@C.
The morphology of the TiOx@C synthesized using different reduction temperatures are shown in the SEM images in Figure 2a-d.
The nanoparticles were dense and chain-structured with the sizes laid within a range between 50 to 100 nm and did not change with the increasing reduction The sizes were bigger than that of initial TiO2, which was 20 nm for the longest axis (Figure 2.
However, they were significantly smaller than that of TiO x synthesized using a reduction temperature of 1050 AC for the same reduction time without carbon encapsulation (Figure 2.
This indicated a successful suppression of TiO 2 sintering by carbon.
Figure 1.
Illustration of the encapsulation mechanism of TiO 2 nanoparticles with carbon.
DOI: https://doi.
org/10.
17509/ijost.
p- ISSN 2528-1410 e- ISSN 2527-8045 239 | Indonesian Journal of Science & Technology.
Volume 6 Issue 1.
April 2021 Hal 235-242 Figure 2.
SEM image of Ti4O7@C nanoparticles synthesized using a reduction temperature of .
800, .
900, .
1000, and .
1050 AC, .
rutile TiO 2 nanoparticles as the starting material, and .
Ti4O7 particles synthesized by reducing TiO 2 at 1050 AC for 30 min without carbon encapsulation.
The core-shell structure of TiOx@C was confirmed from the TEM images of the TiOx@C sample reduced at 1000 AC in Figure 3a-b.
Figure 3a shows carbon layer .
ight colo.
with a thickness of approximately 20 nm encapsulating the core particle .
ark High resolution TEM image of the dark-colored core particle in Figure 3b shows highly crystalline TiOx with clear lattices.
The lattice space was 2.
81 yI which corresponds to Ti4O7 [-1 0 .
It is worth mentioning that the crystallographic shear plane Ae the main characteristic of Magneli phases TiOx Ae is clearly visible in Figure 3b.
The shear planes were neatly ordered as they belong to Ti4O7 phase.
A further increase in the reduction temperature to 1050 AC excessively reduced the TiO2, indicated by the presence of less conductive Ti2O3 phase in the XRD spectrum.
It is interesting that the temperature required for reducing TiO2 to Ti4O7 in this study was slightly less than that reported by other researchers which was usually 1050 AC or higher (Kitada, et al.
, 2012 and Zhang, et , 2.
In the current study, the reducing agents were H2 and carbon.
The small TiO2 size inside the carbon layer and the good contact between carbon and TiO2 surface were believed to facilitate an efficient reduction process.
DOI: https://doi.
org/10.
17509/ijost.
p- ISSN 2528-1410 e- ISSN 2527-8045 Kartikowati, et.
Carbon-coated Single-phase Ti4O7 Nanoparticles as Electrocatalyst Support | 240 Figure 3.
TEM and .
high resolution (HR) TEM of TiO x@C synthesized using a temperature of 1000 AC for the reduction, .
X-ray diffraction spectra of the sample prepared using various reduction temperatures.
Pt nanoparticles were deposited onto the surface of Ti4O7@C with a Pt loading of 15 As shown in the SEM image in Figure 4a, the average Pt size was 3 nm, which was preferred for the electrocatalysis application.
However, the dispersion of Pt nanoparticles was not exceptional compared with the literatures which used the same Pt deposition method, indicated by the wide size distribution and the presence of area with vacant Pt.
The formation of big-sized Pt suggested Pt agglomeration because of two possible reasons.
First, the dense and chain structure of Ti4O7@C nanoparticles could not facilitate a good hydrodynamic of ethylene glycol as the dispersant for Pt precursor during Pt deposition.
Therefore, some parts of Ti4O7@C could not be wetted by the Pt The second possible reason would be the limited available surface for Pt The same tendency was shown in a study by Balgis, et al.
which also involved a computational fluid dynamic simulation (Balgis, et al.
, 2.
Cyclic voltammogram of Pt/Ti4O7@C voltammogram shape of Pt.
The ECSA was calculated using the H2 adsorption charge limited by the potential just above the hydrogen generation .
05 V/RHE) and 0.
V/RHE.
The ECSA was calculated to be 46 m2 mgPt-1.
This value was comparable with the ECSA of commercial TKK catalyst with 20% Pt loading, which was reported to be 44.
13 m 2 mgPt-1.
Further analysis is required to investigate the reason for the good performance despite the low Pt loading.
Speculatively, the good performance may be attributed to the good electron transfer through the Ti4O7@C structure.
Although the 3-aminophenol-derived carbon was reported to be amorphous, the good electron transfer was made possible by the highly crystalline Ti4O7 in the core of the Ti4O7@C.
DOI: https://doi.
org/10.
17509/ijost.
p- ISSN 2528-1410 e- ISSN 2527-8045 241 | Indonesian Journal of Science & Technology.
Volume 6 Issue 1.
April 2021 Hal 235-242 Figure 4.
SEM images of Pt/Ti4O7@C, .
cyclic voltammogram of Pt/Ti4O7@C in 0.
HClO4 solution, and .
cyclic voltammogram of Pt/Ti 4O7@C after several potential cycles from 0 to 1.
4 V/RHE.
Durability of Pt/Ti4O7@C electrocatalyst was evaluated by performing 1000 cycles from 0 to 1.
4 V/RHE.
This condition is considered to be highly severe because the normal theoretical operating potential of a fuel cell does not exceed 1.
2 V/RHE.
Pt/Ti4O7@C electrocatalyst showed low durability after 1000 cycles, that the ECSA decreased from 46 to 15 m2 mgPt-1.
durability study on pure Magneli phases TiO x demonstrated excellent results (Arif, et al.
Therefore, the low durability in the current study is attributed to the carbon oxidation at high potential.
This issue can be overcome by using graphitic carbon to encapsulate Ti4O7 from other carbon sources, for example sucrose.
Ti4O7 phase was formed using a reduction temperature of 1000 AC.
Ptdeposited Ti4O7@C electrocatalyst exhibited an ECSA of 46 m2 mgPt-1.
This value was slightly higher than that reported for TKK commercial electrocatalyst .
13 m2 mgPt-.
and was attained using lower Pt loading .
% versus 20%).
The high performance was speculatively attributed to the highly crystalline Ti4O7 core which promoted good electron transfer.
A room for improvement in the future is available, especially in the catalyst durability, which can be attained by using graphitic carbon for the encapsulation using the same method demonstrated in this ACKNOWLEDGEMENTS
CONCLUSION
A method to synthesize single phase, nano-sized Ti4O7 with carbon layer as electrically conductive anti-sintering agent has been demonstrated in this study.
3aminophenol-derived carbon encapsulated the surface of TiO2 prior to the reduction, through a microwave-assisted hydrothermal method and followed with carbonization.
The resulting particles after reduction.
TiOx@C core-shell, were dense and chain- The authors thank Professor Prof.
Kikuo Okuyama and Dr.
Tohru Iwaki from Hiroshima University for the guidance during the writing of this article.
AUTHORSAo NOTE The authors declare that there is no conflict of interest regarding the publication of this article.
Authors confirmed that the paper was free of plagiarism.
REFERENCES